We can activate an endogenous type C guinea pig oncornavirus (GPV) from any guinea pig cell. We would like to characterize the biophysical, biochemical and biologic properties of endogenous GPV. We hope to determine the role of endogenous GPV in tumor induction and progression. We shall investigate the expression of endogenous virus genes in spontaneous and chemically induced tumors by molecular hybridization and by immunologic techniques. We shall, also, study regulatory mechanism in the expression and repression of the endogenous virus gene.